Flat panel displays are an enabling technology for all contemporary portable consumer electronic devices and large-format televisions. Silicon (Si) crystallization is a processing step that is often used in the manufacture of thin-film transistor (TFT) active-matrix liquid-crystal displays (AMLCDs) and active-matrix organic light-emitting diode (AMOLED) displays. Crystalline silicon forms a semiconductor base, in which electronic circuits of the display are formed by conventional lithographic processes.
Commonly, crystallization is performed using a pulsed beam of laser-radiation that is shaped into the form of a long line having a uniform intensity profile along the length direction (long-axis) and a uniform or “top-hat” intensity profile across the width direction (short-axis). In the crystallization process, a thin layer of amorphous silicon (a “silicon film”) on a glass substrate is repeatedly melted by the pulsed laser-radiation, while the substrate and the silicon layer thereon are translated relative to a source and optics delivering the pulsed laser-radiation. Repeated melting and re-solidification (recrystallization) through exposure to the pulsed laser-radiation, at a certain optimum energy-density, take place until a desired crystalline microstructure is obtained in the silicon film.
Optical elements are used to form the pulsed beam of laser-radiation into a long line on the silicon film. Crystallization occurs in a strip having the length and width of the long line of laser-radiation. Every effort is made to keep the intensity of the pulsed laser-radiation highly uniform along the long line. This effort is necessary to keep the crystalline microstructure uniform along the strip. A favored source of the pulsed laser-radiation is an excimer laser, which delivers laser-radiation having a wavelength in the ultraviolet region of the electromagnetic spectrum. The above described crystallization process, using excimer-laser pulses, is usually referred to as excimer-laser annealing (ELA). The process is a delicate one. The error margin for the optimum energy-density can be a few percent or even as small as ±0.5%.
There are two modes of ELA. In one mode, the translation speed of a panel relative to the laser beam is sufficiently slow that the “top-hat portion” of the beam-width overlaps by as much as 95% from one pulse to the next, so any infinitesimal area receives a total of about 20 pulses. In another mode, referred to as advanced ELA (AELA), the translation speed is much faster and in a single pass over a panel the irradiated “lines” have minimal overlap and may even leave un-crystallized space therein between. Multiple passes are made such that the entire panel is irradiated with a total number of pulses that may be less than in an ELA process to produce equivalent processed material.
Evaluation of crystallized silicon films on panels in a production line is often done off-line, by visual inspection. In particular, panels are checked for undesirable periodic features formed in the silicon film during ELA and AELA processes when the energy density of the crystalizing beam becomes non-optimal. Visual inspection is entirely subjective and relies on highly-trained inspectors, who through their experience are able to correlate observed features in the panels with very small changes in the crystallizing beam, for example, with a less than 1% change in energy-density. In a manufacturing environment, the process of visual analysis to determine if a change of process energy-density is required typically takes between about one hour and one and one-half hours from when the crystallization was performed, with a corresponding adverse effect on the throughput of acceptable panels in a production line.
An on-line method of evaluating crystallized silicon films on panels is described in U.S. Pat. No. 9,335,276, assigned to the assignee of the present invention, and the complete disclosure of which is incorporated herein by reference. In this method a microscope image of a portion of a panel is used for the evaluation. The image is formed from light diffracted from periodic features formed in the recrystallized silicon films by the ELA process. Measured contrast of structure in the diffraction image is one method used to evaluate the annealing process.
A shortcoming of this method is that reflected light from a light source illuminating the panel must be excluded from the microscope objective to provide the diffraction image. This can be done by using a physical stop or by using crossed polarizers between the panel and the microscope. As neither method is completely effective, there is some “softening” or reduction in contrast in the diffraction image. There is a need for a method of evaluating crystallized layers using light diffracted from the layers, wherein the illumination source is de-coupled from a detector measuring the diffracted light.